Current gene therapy strategies for sickle cell disease focus on transduction of hematopoietic stem cells with viral vectors containing anti-sickling globin genes such as y-, 0/5- or modified P-globin genes. Although this is a viable approach, long term expression of therapeutic levels of globin mRNA and protein from transduced genes magamma be, difficult to achieve. An alternative approach is to transduce stem cells with a transcription factor gene encoding a protein that specifically enhances endogenous gamma- or beta-globin gene expression. In this case relatively low levels of a novel transcription factor may stimulate high levels of gamma- or 5-globin gene expression. Sangamo BioSciences is a company which has pioneered the use of zinc finger proteins (ZFPs) of the Cys2-His2 type with high specific affinities to almost any regulatory sequence of interest. Designer ZFPs have more robust characteristics than endogenous factors. ZFPs are designed to bind to virtually any sequence with high specificity and greater affinity than endogenous factors. ZFPs can also be designed to integrate functions normally distributed between several endogenous factors, which would otherwise have to be added combinatorially to effect a desired outcome. ZFPs that specifically and efficiently activate gamma- and beta-globin gene expression in cultured cells will be tested in a mouse model of sickle cell disease. These mice switch from human fetal hemoglobin (HbF) to human sickle hemoglobin (HbS) after birth in a manner similar to human patients with the disease. Although the mice are asymptomatic at birth, severe disease develops as the switch occurs, and adult animals develop most if not all of the pathology of the disease. ZFPs that specifically and efficiently activate gamma- and beta-globin gene expression will be transduced with lentiviral vectors into purified hematopoietic stem cells isolated from the sickle mice. After transplantation of these genetically modified cells into recipients, the animals will be monitored for long term correction of the disease. Relatively low levels of these potent transcription factors should be capable of activating gamma- and beta-globin genes to a level that will ameliorate the pathology observed in most organ systems. These studies will provide a solid foundation for subsequent clinical trials in human patients with this devastating disease.
